Manufacture of dispersion strengthened lead by screw extrusion of oxide-coated particles



July 1968 s. T. GAZZARD 3,393,069

MANUFACTURE OF DISPERSION STRENGTHENEI) LEAD BY SCREW EXTRUSION OP OXIDE-COATED PARTICLES Filed NOV. 10, 1965 Transverse Section Through Rum-Extrusion INVENTOR SIMON THOMAS GAZZARD ATTORNEY 3 United States Patent 3,393,069 MANUFACTURE OF DISPERSION STRENGTH- ENED LEAD BY SCREW EXTRUSION 0F 0X- IDE-COATED PARTICLES SllllOIl Thomas Gazzard, Harlow, England, assignor, by mesne assignments, to St. Joseph Lead Company, New York, N.Y., a corporation of New York Filed Nov. 10, 1965, Set. N 0. 507,204 Claims priority, application Great Britain, Nov. 10, 1964, 45,776/ 64 7 Claims. (Cl. 75--206) ABSTRACT OF THE DISCLOSURE The specification discloses the preparation of dispersion strengthened lead by a process which comprises contlnuous screw extrusion of lead oxide coated particles of lead, preferably containing from about 0.1 to about 10% by weight lead oxide calculated as PbO.

Starting with lead powder the individual particles of which have a surface layer of lead oxide, dispersion strengthened lead can be produced by subjecting the lead powder to a mechanical deformation which breaks up the oxide layers and disperses the resultant oxide fragments throughout the lead matrix so that there is obtained a coherent lead product the strength of which, due to the dispersed oxide fragments, is substantially greater than that of ordinary lead. The lead powder can simply be extruded in order to subject it to the mechanical deformation referred to (such deformation being an inherent result of the extrusion action) but hitherto, so far as we are aware, such extrusion has always been effected in an extrusion press of the ram type. This would be natural, seeing that one is dealing with the lead in a solid rather than liquid form, it being appreciated that the lead is not changed to the molten state either before or during its extrusion.

We have now ascertained, however, that not only is it possible to extrude the oxide-coated lead powder whether loose or in pre-consolidated pellets, in an extrusion machine of the screw type, but that unexpectedly the strength and ductility of the-resultant lead product can be significantly greater, and the extent of air entrapment in it significantly less, than for the product obtained by extrusion in a ram type extrusion press. Moreover, the screw extruded product exhibits a more uniform dispersion of the lead oxide, with reduced directionality, presumably due to the high shear conditions experienced.

, Screw extrusion has the further advantage that it can be used as a continuous process and this can be of particular importance when long lengths are required, for instance in the case of lead tubing or lead cable sheathing. In extruding long lengths with a ram press (namely lengths requiring more than a single charge of the press) it is diflicult to ensure that at the boundary between one charge and the next the lead maintains its mechanical characteristics and is free of entrapped air. Screw extrusion starting with lead in its molten state is, of course, already used in the cable industry. The molten lead employed can have another material such as antimony alloyed with it so as to increase its mechanical strength, but this leads to problems in the screw extrusion process due to a tendency for the alloying constituent to segregate in the melt. When lead powder based on substantially pure lead (as distinct from alloyed lead) is extruded in accordance with the invention, the strength of the resulting product can be commensurate with or indeed exceed that of ordinary alloyed lead as used for cable sheathing. The use of lead powder based on alloyed lead, e.g. with 0.2% antimony, could be expected to give even greater strengths and segregation in the screw extrusion machine is unlikely to occur. Therefore hereinafter and in the claims lead is ice 2 to be understood as connoting both alloyed and unalloyed lead.

It seems probable that dispersion strengthened lead made in accordance with the invention may also be superior in other mechanical properties such as fatigue, impact strength and creep resistance, and possibly superior in corrosion resistance, but we have not yet investigated these characteristics.

The lead powder used for screw extrusion in accordance with the invention preferably has a controlled leadoxide content in the range 0.1% to 10% (or possibly higher) calculated as PhD in terms of weight. The strength of the resultant product will vary with the particular oxide content present in each particular case, and will also depend on such factors as the original particle size of the lead powder and on the fineness of the dispersed oxide and the inter-particle spacing of the oxide fragments in the extruded product, these last two factors depending on the amount of shear deformation to which the extrusion action subjects the powder and on whether and to what extent the powder is previously subjected to a mechanical treatment which produces a preliminary fragmentation and dispersal of the lead oxide. Thus the loose powder may be subjected in an oxidising atmosphere to a ballmilling treatment which results in breaking up of the oxide and forcing of the fragments into the lead particles accompanied by further oxidation of freshly exposed surfaces. Alternatively or in addition if the powder is to be consolidated before feeding to the screw extruder the pre-consolidation may be effected by a technique for example extrusion, which imposes such forces of deformation during the consolidation that it is accompanied by fragmentation and dispersion of the oxide in the consolidated mass.

Experiments have been performed for comparison purposes with a ram extrusion press and with a screw extrusion machine having a low-clearance solid screw arranged to feed the powder forward from the barrel of the machine through a short chamber and a /2 inch diameter die, the barrel, and correspondingly the screw, having a main constant diameter portion of about 1 /2 inches diameter preceded by a convergent tapered portion giving early compaction with maximum elimination of air. Because only a limited extrusion force was possible with the particular screw machine used, the extrusion was performed at an elevated temperature with lead powders having a relatively low oxide content. With a stronger press, however, extrusion with powder of higher oxide content and at lower temperatures, probably down to room tempera'ture (20 C.) should be possible. The experiments were performed with three different powders, identified as powders A, B and C. In each case the total extruded lengh was cut into suitable lengths for tensile strength and elongation tests. Powder C, prior to the comparative extrusions, was pre-consolidated by extrusion and then broken up into pellets of about half a gram in weight for feeding into the ram and screw extruders.

Powder A Particle size 35 microns maximum, 60% w./w. between 30 and 50 microns. Lead oxide content (calculated as PbO) 2.2% (approx.) w./w. Lead carbonate content 0.2% (approx.) w./w. Length extruded:

Screw 10 feet.

Ram 6 feet. Extrusion temperature 200 C. Extrusion ratio (ram) 20:1.

Average speed of extrusion (screw) 4.8 inches/minute.

Ram Screw Average tensile strength (lb./in. 2, 600 3,010 Average elongation (percent on gauge length of 4 /A) 31 35 A=eross sectional area of test specimen. 5

Powder B Particle size As for powder A. Lead oxide content 1.6% (approx) w./w. Lead carbonate content 0.3% (approx) w./w. Length extruded:

Screw 10 feet. Ram 6 feet. Extrusion temperature 190 C. Extrusion ratio (ram) 20:1. Average speed of extrusion (screw) 4.5 inches/minute.

Ram Screw Average tensile strength (lb./in. 2,800 3,030 Average elongation (percent on gauge length of4 VA.) 46

While the tapering in the screw extrusion machine had been provided with a view to minimising air entrapment, tests on the extrusion products of powder B showed that the extent of air entrapment in the screw extruded product was significantly less than expected. On reheating the extruded samples (causing expansion of any ent-rapped air) the degree of swelling of the screw extruded product was found to be about ten times less than with the ram extruded product. In particular, samples extruded from powder B after immersion ten times in a salt bath at 260 C., each time for a period of 22 seconds to allow the sample to reach a temperature of 250 C., showed resultant decreases i-n density (a measure of the extent of swelling and thence of air entrapment) of about 0.008 gram per cubic centimeter for the screw extruded product as against about 0.079 for the ram extruded product. The initial density of the screw extruded product was about 11.32 grams per cubic centimeter.

and because of uncertainty as to the temperature for the screw extrusion, a series of ram extrusions at different temperatures was performed. Comparison of samples having approximately equal ultimate tensile strengths of about 3100 lb./ in. gave the following result:

Ram Screw Elongation (percent on 4 JK) 36 41 Reduction in area b9 85 The advantage of screw extrusion over ram extrusion as regards efliciency of oxide dispersal is particularly important when for ease of handling, preheating (if desired) and charging into the extruder, the use of preconsolidated power is preferable. This advantage is clearly demonstrated by the accompanying line drawing reproductions of microphotographs of polished transverse sections of a screw-extruded sample and a ram-extruded sample (FIGS. 1 and 2 respectively) taken from those produced from powder C. The photographs show the greatly improved homogeneity of the screw-extruded product as compared with ram-extruded, due to the much more nearly complete destruction of inter-pellet interfaces and dispersion of oxide which result from the' high shear deformations impressed by the screw-extrusion.

What I claim is:

1. A process for the manufacture of dispersion strengthened lead which comprises continuous extrusion of lead oxide coated particles of metallic lead in a screw type extrusion apparatus.

2. In the manufacture of dispersion strengthened lead according to claim 1 the step of pre-consolidating the lead powder by extrusion and dividing the product of this extrusion into pellets which are fed into the screw type extrusion machine.

3. In the manufacture of dispersion strengthened lead according to claim 1, the step of pro-subjecting the lead powder to a mechanical treatment which produces a preliminary fragmentation and dispersion of the lead oxide.

4. Manufacture of dispersion strengthened lead according to claim 3 wherein as such mechanical treatment the lead powder is subjected in an oxidising atmosphere to a ball-milling treatment which breaks up the lead oxide and forces the resultant oxide fragments into the lead of the powder particles while further oxidation takes place on the freshly exposed lead surfaces.

5. Manufacture of dispersion strengthened lead according to claim 3 wherein as such mechanical treatment the lead is subjected to a pro-consolidation step which imposes such forces of deformation during the consolidation as to fragment the lead oxide and disperse it in the consolidated mass.

6. Manufacture of dispersion strengthened lead according to claim 1, wherein the screw extrusion is performed at an elevated temperature which is between room temperature and the melting point of the lead.

7. Manufacture of dispersion strengthened lead according to claim 1 from lead powder having a lead oxide content in the range 0.1% to 10% by weight calculated as PbO.

References Cited UNITED STATES PATENTS Re. 18,710 1/1933 Dunsheath 72-262 2,378,539 6/1945 Dawihl 75-203 X 3,189,989 6/1965 Ebdon 29-4205 3,254,143 5/1966 Heitman 264-29 3,284,372 11/1966 Bailey 264-29 3,315,342 4/1967 Roberts 75-206 X 3,320,664 5/1967 Krantz et a1. 75-206 X FOREIGN PATENTS 970,259 9/ 1964 Great Britain.

OTHER REFERENCES Roberts et al., Dispersion-Strengthened Lead and Its Applications. Powder Metallurgy, 1962, No. 10, pages 132157.

BENJAMIN R. PADGE'IT, Primary Examiner.

L. DEWAYNE RUTLEDGE, CARL D. QUARFORTH,

Examiners.

R. L. GRUDZIECKI, Assistant Examiner. 

